Experimental methods for following the rate of a reaction

Quenching

We can stop a chemical reaction in its tracks using a practical technique called quenching. There are two ways to go about this:

Add a reagent to the reaction mixture to ‘neutralise’ one of the reactants so that the primary reaction can no longer proceed
Plunge the reaction mixture into a known volume of ice-cold water to slow the reaction down to the point at which it has effectively stopped

The reaction between propanone and iodine is catalysed by hydrogen ions.

CH₃COCH_3(aq) + I_2(aq) ⇾ CH₃COCH₂I_(aq) + I^–_(aq) + H⁺_(aq)

We can follow the progress of this reaction by determining how much iodine is present in the reaction mixture at set time intervals. To do this we must remove a small portion of the reaction mixture and add it to an excess of sodium hydrogen carbonate which reacts with (neutralises) the acid catalyst present and halts the reaction. We can then titrate the iodine in the sample against a standard solution of sodium thiosulphate with a starch indicator.

Let’s look at a sample practical method …

It is really important to detail the precision with which we need to make each volume measurement when describing experimental methods. Volumetric pipettes are precise to within 0.1cm³ and burettes to 0.05cm³, so be sure that this is clear e.g. 50.0 cm³ of 0.02 mol dm^-3 iodine solution measured using a 50.0cm³ volumetric pipette. This is the kind of attention to detail expected for a grade A – writing ‘measure out 50cm³ of iodine’ is not enough.

We need an excess of the sodium hydrogen carbonate solution to quench the reaction so it does not need to be measured out precisely, a measuring cylinder will do. This is implied in the diagram above as I have noted that the flask contains 25cm³ not 25.0cm³.

Obviously one sample is not going to be enough! We would remove a 10.0cm³ sample of the reaction mixture at a number of set time intervals (maybe 5 altogether) and pipette each into a clean conical flask containing sodium hydrogen carbonate, and then titrate against sodium thiosulphate.

2Na₂S₂O_3(aq) + I_2(aq)→ Na₂S₄O_6(aq) + 2NaI_(aq)

We can use the titration results to find the concentration of iodine remaining in the reaction mixture at each time interval, and then plot a graph of [I₂] v. time. The gradient of this graph at t = 0 gives us the initial rate of reaction for this particular initial concentration of iodine (0.02 mol dm^-3).

By repeating the experiment with different initial concentrations of iodine solution (keeping the propanone and hydrochloric acid concentrations constant) we could eventually plot a graph of [I₂] v. initial rate of reaction, from which we would be able to deduce the order of the reaction with respect to iodine.

Measuring gas volumes

If the product of a reaction is a gas, this can be collected in either a gas syringe or in an upturned burette under water.

If the volume of gas is measured at set time intervals, we can determine the number of moles of gas and hence the concentration of the reactants at each time. A graph is then drawn of the concentration of a reactant v. time and the gradient at t=0 is the initial rate of reaction. Alternatively, we could simply plot a graph of volume of gas v. time and take the gradient at t=0 to be equivalent to the initial rate of reaction.

A second method involves measuring the time taken for a fixed volume of gas to be reached, in which case the initial rate of reaction is taken to be equivalent to 1/t (exactly as we do in a clock reaction).

In either case the experiment is then repeated with varying initial concentrations of the reactant being studied so that a set of results is obtained that would allow us to plot a graph of initial rate of reaction v. concentration of reactant, from which we could determine the order of reaction with respect to that reactant.

Exam tips for describing experimental methods

If you are asked to devise a method for any experiment / practical procedure in an exam you need to describe the fine detail in order to pick up the highest marks.

It is generally easier to draw and label the set up for apparatus rather than try to describe it, but your drawing has to be accurate!

Fine detail includes describing how you will add one reactant to another, in terms of both precise concentrations, volumes (e.g. 25.0cm³ rather than 25cm³) and the pieces of equipment needed for each measurement (burette, volumetric pipette, graduated pipette, measuring cylinder). If you are measuring the mass of a solid, to how many decimal places? State when stop watches are started and stopped.

How many measurements do you need to make? Repeats? Explain how will you process your results – describe any calculations you will need to carry out and how to draw any graphs (including labelling axes and where to draw gradients etc.).

For more tips on describing titrations check out this post!